Aromatic carboxylic acids are usually produced in an oxidation reactor by subjecting alkyl aromatic compounds as raw materials to liquid-phase oxidation in a solvent containing acetic acid in the presence of a catalyst containing a heavy metal compound and a bromine compound by use of a gas containing oxygen in the form of molecules. During this oxidizing reaction, water is produced. Reaction vapor discharged from the oxidation reactor and a mother liquid of aromatic carboxylic acid slurry contain reaction water. Such reaction vapor and mother liquid, the latter being obtained by separating aromatic carboxylic acids from the slurry produced, are typically dehydrated and distilled to recover acetic acid, which is recycled as a solvent for oxidation reaction.
Since water and acetic acid are difficult to separate from each other and because a dilute aqueous solution of acetic acid in particular has a specific volatility near 1, in order to separate acetic acid from water by dehydration and distillation so that the concentration of acetic acid in the distilled water will not exceed 1 weight percent, it is necessary to increase the number of stages of the distilling column and/or increase the reflux ratio. This increases the size of the dehydration/distillation column, which in turn pushes up its cost as well as the facility cost. Also, because the latent heat of vaporization of water is high, reboiler loads of distillation increases by increasing the reflux ratio. Various measures have been reported for avoiding this problem. For example, patent document 1 reports a method of reducing distillation loads by combining distillation with extraction. Patent document 2 reports that it is possible to reduce the reflux ratio and energy consumption by azeotropic distillation using an azeotropic agent in the dehydration/distillation column.
Also known are methods of dehydration in which a separation membrane such as a reverse osmosis membrane is used in distillation (patent documents 3 and 4). Patent document 3 proposes to remove water and alcohol by separation using a membrane in producing an aromatic carboxylic acid. With this arrangement, in which distillation is combined with separation using a membrane, it is possible to reduce loads on the dehydration/distillation column, so that an aliphatic carboxylic acid, a solvent, can be recovered with high efficiency.
The alcohol mentioned in patent document 5 is an unnecessary byproduct produced when recovering acetic acid by hydrolyzing an aliphatic carboxylic acid ester such as methyl acetate, which is a byproduct produced in the system. It is separated together with water by a membrane.
Patent document 1: JP patent publication 7-53443
Patent document 2: WO96-06065
Patent document 3: JP patent publication 2001-328957
Patent document 4: WO02-50012
Patent document 5: JP patent publication 2001-328957